1. Field of the Invention
This invention relates to gas turbine units, or gas turbine engines, and more particularly to annular diffusers in gas turbine units. More particularly still, this invention relates to annular diffusers located downstream of gas turbines in gas turbine units.
2. Description of the Prior Art
A gas turbine unit used in power generation usually comprises two shafts: a high pressure shaft of the gas generator portion and a low pressure shaft of the load portion to assure greater application flexibility. The gas generator portion of the gas turbine unit consists of a compressor, which in general is of the axial-flow type, and which is directly coupled to a high pressure turbine having usually one or more axial-flow stages. The load portion of the gas turbine unit has one or more low pressure axial flow turbine stages and is directly coupled to a load, which may be, for example, an electric power generator, a compressor or a fan, or a ship propulsion shaft.
In most gas turbine units air enters the compressor of the gas generator portion from which it flows to one or more combustion chambers. Subsequently, the products of combustion, or combustion gases, flow through the high pressure turbine of the gas generator portion which provides power needed by the compressor, the high pressure turbine being followed by an annular diffuser. From the high pressure turbine the combustion gases flow to the low pressure (or hot end) turbine which is also followed by an annular diffuser.
A diffuser designed for a gas exiting at subsonic speeds from the high-pressure and the low-pressure turbines of a gas turbine unit is a duct whose cross-sectional area increases with distance. A well-designed and well-operating diffuser located after a turbine produces lowering of pressure downstream of the turbine which results in an increase of energy available to the turbine to do work and, therefore, in an increase of the efficiency of the whole gas turbine unit. A diffuser produces lowering of pressure downstream of the turbine by diffusing, or slowing down the flowing gas so that a significant amount of the kinetic energy of the gas is converted into enthalpy with an accompanying increase of pressure.
Almost all tests on the pressure recovery, or performance, of diffusers reported in the literature were run with uniform flow at inlet to the diffusers and under incompressible flow conditions.
The main feature which differentiates the flows in diffusers located at the exit of turbines from the vast majority of diffuser flows is the presence of wakes at diffuser inlets. The wakes develop at the trailing edges of blades as a result of coalescence of low velocity boundary layer flows which form adjacent to blade surfaces. Such wakes can become very thick when flow separation from the blades takes place. The wakes which have the largest effect on diffuser performance come, in general, from the turbine blades located just upstream of the diffuser inlet. Other, weaker, wakes may also enter a diffuser, for example the traces of wakes produced by the stationary nozzles located upstream of the final blades. (A smaller number of wakes can also be produced by the flow straightening vanes sometimes placed ahead or upstream of diffusers not far from the diffuser inlet.) All these wakes enter the diffusers located downstream of the turbines where such wakes decay. The process of decay of wakes involves entrainment of the surrounding fluid into the wakes resulting in a decrease of its velocity, which decrease in velocity is accompanied by a pressure rise in the diffuser, or flow diffusion. Such pressure rise occurs in addition to the flow diffusion and pressure rise caused by the increase of the diffuser cross-sectional area. The decay of wakes thus produces a secondary diffusion in addition to the primary diffusion incident to increase of the diffuser cross-sectional area and is discussed in applicant""s application Ser. No. 09/366,478 for patent filed Aug. 3, 1999 and entitled xe2x80x9cExhaust Flow Diffuser for a Steam Turbine,xe2x80x9d the disclosure of which is hereby incorporated by reference in the present application.
Until now, annular diffusers of gas turbines have been designed using results from performance tests on diffusers having uniform flow distribution at the inlet and in general for incompressible fluid flows, with the results corrected for the effect of compressibility of the fluid on the area ratio-pressure rise relationship. The correction for compressibility decreases the allowable rate at which the diffuser cross-sectional area can increase for optimal diffuser performance compared to that of an incompressible flow. (At the optimal performance conditions there is no permanent flow separation from the walls which would cause severe deterioration of diffuser performance.) What has not been recognized until now is the fact that the process of decay of wakes has a large effect on the allowable rate of increase of the diffuser area ratio. Permanent flow separation in a diffuser occurs when the (longitudinal) pressure gradient becomes too large. The decay of wakes produces flow diffusion with accompanying pressure rise, or pressure gradient, independently of the pressure gradient which is produced by the diffuser cross-sectional area increase and this secondary diffusion is additive to the primary diffusion incident to cross-sectional area increase. In order to keep the pressure gradient in a diffuser flow with wakes at the same magnitude that existed or exists in a uniform flow, the rate of increase of cross-sectional area of the diffuser has to be decreased correspondingly. This is true in the initial section of a diffuser in which most of the decay of wakes takes place. After the wakes have become almost completely dissipated, the rate of diffuser cross-sectional area increase should correspond to the optimal rate of increase determined for an incompressible uniform flow corrected for the effect of compressibility of the fluid.
The prime object of this invention is to maximize the amount of work or power delivered by a gas turbine unit operating at design and off-design conditions by lowering the pressure at the exit of the hot end and cold end turbines.
It is a further object of the invention to provide a diffuser which will not induce permanent flow separation from the walls which would increase pressure at the turbine exit and decrease the amount of power produced by the turbine.
It is still a further object of this invention to account for the effect of wakes naturally occurring in gases passing from turbines through diffusers on the diffusion process in the diffuser.
It is a still further object of the invention to provide a diffuser having parameters that will account for the compressibility of flowing gases as well as for the effect of wakes.
It is a still further object of the invention to provide a diffuser having a limit placed on the rate of increase of its cross-sectional area in the initial portion of the diffuser.
It is a still further object of the invention to limit the increase in the cross-sectional area of a diffuser in the initial portion of the diffuser extending to one half the diffuser inlet height to no more than 6.5 percent of the inlet cross-sectional area corresponding to a two-dimensional straight-wall diffuser angle of 3.7 degrees or less.
Additional objects and advantages of the present invention will become evident from review of the following specifications and appended drawings.
The process of decay of wakes inherent in the exhaust flow diffusers of turbines produces a certain amount of diffusion, and therefore also a pressure gradient, which adds to that which results from the increase of the diffuser cross-sectional area. In order to keep the magnitude of the pressure gradient the same as in the case of a flow having uniform inlet conditions (without wakes) with an optimal amount of diffusion so as to avoid permanent flow separation from diffuser walls, the rate of increase of diffuser cross-sectional area must be correspondingly smaller.
To allow for the presence of wakes at the diffuser inlet, a limit is placed on the initial rate of increase of diffuser cross-sectional area. In the diffuser of this invention the initial increase of the cross-sectional area of the diffuser is limited to less than a predetermined fraction of the inlet cross-sectional area for a certain distance from the inlet. In accordance with this invention the area increase in the diffuser cross-section from the inlet to a distance downstream from the inlet of one half of the diffuser height at inlet, measured along the diffuser mean line, is limited to no more than 6.5 percent of the inlet cross-sectional area. This represents a value of the corresponding angle of a two-dimensional straight-wall diffuser of 3.7 degrees. By limiting the increase in cross-sectional area of the diffuser to such percentage increase in the initial portion of the diffuser as defined, the wakes are allowed to substantially dissipate without producing flow separation from diffuser walls after which the cross-sectional area of the diffuser may be increased at a higher rate consistent with a separation-free flow and corresponding mainly to the optimal rate determined for an incompressible and uniform flow corrected for the effect of compressibility of the fluid.